This invention relates to an improved single pass dialystate supply apparatus employing a pair of cylinders equipped with hydraulically driven diaphragms, and to a method for controlling the diaphragms and resultant dialysate flow to and from a hemodialyzer during hemodialysis. The apparatus of this invention is of the general type disclosed in copending application Ser. No. 958,329 now U.S. Pat. No. 4,209,391, entitled Apparatus and Method for Automatically Controlling Hemodialysis at a Pre-selected Ultrafilatration Rate which is assigned to the assignee of this application. The apparatus of this invention provides new means for providing flow of identical liquid volumes of dialysate to and from the kidney so as to attain the same general method of hemodialysis treatment that is achieved by copending application serial No. 958,329; the new means are modified and improved relative to the power driven pistons or diaphragms employed in that apparatus; the new means are also simpler and cheaper to manufacture and operate than the means of the prior disclosed apparatus.
The circuit of application Serial No. 958,329 employs a common power driven piston rod that is attached to pistons, or diaphragms, in each of two substantially equal volume cylinders. Oscillation of the piston rod causes dialysate to be supplied to and removed from a hemodialyzer on an alternating basis from one of the two cylinders while the other cylinder is being filled with fresh dialysate as spent dialystate in that same cylinder is being sent to drain. Heretofore, systems using a pair of matched volume output pumps have been commonly referred to as closed system and one such system is disclosed in U.S. Pat. No. 4,021,341. In the system of U.S. Pat. No. 4,021,341 the assumption is made that by employing inlet and output pumps operating at the same speeds, the volume of dialysate ot the kidney is the same as the volume of dialysate sent to drain and any excess volume in the line from the kidney represents water extracted from the blood in the kidney. This assumption has proved to be incorrect and systems relying on measurements of such excess fluid volume as the ultrafiltrate control mechanism have proved to be inaccurate and commercially unsatisfactory. The source of error in the equal volume assumption employed in connection with prior art closed systems was identified in application Ser. No. 958,329 and means were provided in its new circuit which solved the problem and eliminated the error in the false assumption by permitting only bubble-free dialysate to enter each of the two cylinders. As the result, the new circuit of application Serial No. 958,329 enabled control of ultrafiltrate rate merely by pre-setting the rate of bubble-free liquid withdrawn from the circuit through the use of an independently controlled ultrafiltrate withdrawal means such as a third piston or a volumetrically controllable pump.
The apparatus of this invention employs hydraulically driven diaphragms in lieu of the power driven pistons or diaphragms; the circuit of this invention is otherwise generally similar to that of Ser. No. 958,329. Early attempts to merely substitute hydraulic drives for positive mechanical drive in the diaphragm embodiment of that circuit were failures. It was found that flexible diaphragms passively driven in response to higher fluid pressure on one side of the diaphragm than on the opposite side faced an additional control problem due to fluctuations or variations in rate of traverse within the cylinder that were not encountered with positively driven diaphragms. It was observed that traverse rate fluctuations were variable with time and uncontrollable because the hydraulic driving pressure on the infeed dialysate was either the dialysate mixing pump pressure or water source line pressure and each such pressure is subject to unexpected variations; in contrast, the driving force in the spent dialysate line of the circuit is mechanically induced hydraulic pressure which results from pump operation that is likewise subject to sporadic variation. Summarily stated, it was found that the hydraulic driving force on the diaphragms in each of the two cylinder units for an entire cylinder length traverse was rarely, if ever, identical. These driving force variations cause the passively driven diaphragms in each of the cylinders to arrive at the end of stroke at different instants in time, with the absolute time difference varying as a function of the pressure variations during a given traverse from one side of the cylinder to the other. Whereas positive drive common to the two cylinders insured simultaneous attainment of end of stroke of the diaphragms in the fresh/drain and kidney/spent circuits, it became necessary with the hydraulically driven diaphragms to elect to switch valves at the instant of end of stroke of either the first or the second diaphragm. Attempts to switch at the instant of the first to arrival at end of stroke failed because of progressive shortening of each stroke; this procedure culminated in more and more rapid switching from cycle to cycle until final arrival in a flutter lock-up condition which is unsatisfactory. Switching at the instant of the second, or latest, end of stroke of the diaphragms in the two cylinders in the separate flow paths produces a no-flow condition of fluid in the circuit of the first to arrive diaphragm during the interim until the second diaphragm achieves its end of stroke.
It was found that when the no-flow condition occurred in the infeed, fresh dialysate supply line, undesirable effects resulted in the dialysate make-up portion of the single pass dialysate circuit which supplies the bubble-free fresh dialysate to the circuit of this invention. Alternatively, when the no-flow condition occurs in the spent dialysate removal line the flow of dialysate in the dialyzer is interrupted and diffusion rates across the semipermeable membrane are detrimentally affected. Based upon the undesirable aspects of these options it was found to be necessary to control the traverse of the diaphragms in each of the two cylinders such that the instant of end of stroke in each is synchronized.
The apparatus of this invention provides modified circuit elements and associated control means which achieve the necessary control of diaphragm traverse to attain the needed synchronization to enable successful commercial use of hydraulically driven diaphragms in hemodialysis treatments.
No more pertinent prior art to this invention is known than that discussed in the specification of application Ser. No. 958,329 and that disclosure is hereby incorporated into this specification. An automated two constant volume displacement pump system for supplying dialysate to and from a hemodialyzer is disclosed in U.S. Pat. No. 4,113,614. In that system the assumption of equal pumped volumes to and from the hemodialyzer is relied upon and is erroneous and inaccurate because of failure to remove bubbles from the spent dialysate prior to arrival at the dialysate effluent pump. Additionally, one embodiment of U.S. Pat. No. 4,113,614 employs a dialyzer having filtration characteristics that are not constant and known and to establish a near constant ultrafiltration rate in that embodiment presets a transmembrane pressure and during hemodialysis measures the quantity of fluid in excess of that pumped out as effluent and automatically adjusts the transmembrane pressure in response to variations in rate of ultrafiltrate generation. In comparison, the present invention does not preset transmembrane pressure, but does preset ultrafiltrate withdrawal rate.
A second embodiment of U.S. Pat. No. 4,113,614 employs a dialyzer having filtration characteristics that are constant and known; in this embodiment ultrafiltration rate is preset and the quantity of fluid in excess of that pumped out as effluent is measured and differences in the rate of ultrafiltrate generation are used to generate signals which control the pressure in the system. In contrast, this invention uses a hemodialyzer having filtration characteristics that are not constant. This invention makes no attempt to control pressure in the system and relies upon fluid withdrawal to produce inherent pressure conditions to generate from the blood the quantity of water that equals the withdrawn quantity of dialysate; this invention maintains the hydraulic integrity of the entire circuit by insuring that the precise quantity of bubble-free dialysate fed to the kidney does equal the quantity of bubble-free dialysate that is sent to drain, and withdraws only bubble-free ultrafiltrate. Moreover, and as above mentioned to avoid dialysate make-up undesirable effects or irregular flow rates through the kidney, this invention monitors fluid flow rates in the fresh/drain and in the kidney/spent circuits and employs the difference in time of arrival of each of the diaphragms in the two cylinders at its end of stroke to generate signals that in turn are used to change the rate of dialysate flow in the spent dialysate removal line to thereby synchronize the end of stroke instant for the two diaphragms.